1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine and the like, and an optical scanning apparatus used in the image forming apparatus.
2. Related Art
Hitherto, a tandem-type color image forming apparatus that performs latent image writing, which is formation of an image on an image bearing member such as a drum by a light beam, separately by optical scanning apparatuses provided respectively for image bearing members is known. In such an optical scanning apparatus, optical deflectors such as polygon mirrors and motors for driving them are needed in a number of light sources. However, an optical scanning apparatus using such an optical deflector is relatively expensive. Thus, to individually dispose an optical scanning apparatus at each image bearing member causes a problem of a component cost and a manufacturing cost.
Thus, in order to reduce costs and the size of a tandem-type color image forming apparatus, such an image forming apparatus is known that as means for performing optical scanning for a plurality of image bearing members, an optical deflector is made common, and light beams from a plurality of light sources are simultaneously deflected and scanned by one optical deflector so that the light beams from the plurality of light sources are simultaneous radiated to a plurality of image bearing members by one optical deflector, and optical scanning is performed.
For example, an optical scanning apparatus in FIG. 6 is provided with a plurality of (four) scanning optical systems. Each of the plurality of (four) scanning optical systems is provided with a plurality of light sources (not shown), an optical deflector 62 that reflects light beams from the plurality of light sources in two symmetrical directions and deflects them, a first image-forming lens 63 disposed in a reflection direction of the optical deflector 62 and through which the light beam reflected by the optical deflector 62 passes, mirror groups 65, 75, and 76 that guide the light beam having passed through the image-forming lens 63, a photoreceptor drum 2 on which the light beam having passed through the mirror groups 65, 75, and 76 forms an image, and a second image-forming lens 70.
In such an optical scanning apparatus, if a distance from the light source to the first image-forming lens 63 and a focal distance of the first image-forming lens 63 are determined, an optical path length from the first image-forming lens 63 to the photoreceptor drum 2 becomes constant. Thus, if size reduction of the apparatus is to be realized by arranging the photoreceptor drum 2 close to the optical deflector 62, the optical path needs to be folded back.
Thus, in the optical scanning apparatus shown in FIG. 6, the mirror groups 65, 75, and 76 are arranged between the first image-forming lens 63 and the photoreceptor drum 2, whereby size reduction of the device is promoted by folding back the optical path.